1. Field of Invention
The invention relates to a display arrangement and to a method of driving such a display arrangement.
2. Description of the Related Art
Standard color LCD (Liquid Crystal Display) panels are normally controlled digitally and are therefore unable to display more than eight basic color combinations (two intensity levels per color, that is red, green, and blue). In order to overcome this limitation it is known to switch a pixel quickly, that is at a rate greater than the frame refresh rate, between its two intensity levels to artificially generate intermediate intensity levels. This, however, introduces further problems. The simplest way of implementing the faster switching of pixels is to use a frame wide pulse width modulation arrangement but this requires an increase in frame rate by a factor equal to the required number of intensity levels. In addition there are limitations in the performance of typical display panels, such as the maximum shift clock rate and increased power consumption. In addition a higher frame rate requires a proportionate increase in the frame buffer bandwidth. It is possible to use static dithering techniques to imitate a higher color depth. While this option works well with high resolution static images such as in color printing, the larger pixel size associated with an LCD panel combined with the high contrast between adjacent pixels produces a high visible noise in the displayed image.
One of the properties of LCD panels is the relatively slow response of the crystals to changes in the applied signal. The switching times of each LCD pixel can be of the order of tens and even sometimes hundreds of milliseconds. This behavior improves the performance of a pixel switching algorithm, but switching complete frames is still too noticeable at low frame rates.
It is known that the visibility of the flicker is dependent on the area of the flickering surface. Consequently, using different switching patterns for adjacent pixels can significantly reduce the flicker. The human brain is, however, highly specialized in pattern and shape recognition and as a result regular patterns in space and in time are very noticeable, usually as moving or trembling structures.
To overcome these problems the use of a pseudo-random noise source as the basis of a dynamic dithering scheme has been proposed and described in U.S. Pat. No. 5,703,621. The arrangement described uses a two dimensional error propagation scheme in order to produce correct shading for narrow vertical structures. While this is satisfactory as far as the displayed image is concerned it has the disadvantage of requiring complex hardware and/or software for its implementation. Consequently it is a relatively expensive solution.
A much less expensive solution would be possible if a one dimensional error propagation scheme was used. This, however, results in an inability to produce correct shading for narrow vertical structures.
It is an object of the invention to enable the production of images using an LCD having a greater number of levels of resolution than that provided by the pixel switching levels using dynamic dithering which is less expensive to implement than known two dimensional error propagation schemes.
The present invention provides a display arrangement comprising a flat panel display device having an array of pixels, and a driving circuit arrangement for switching each of the pixels between two intensity levels at a rate greater than the frame refresh rate so as to produce the visible effect of intermediate intensity levels, wherein the driving circuit comprises a plurality of intensity level error registers, one for each intermediate intensity level; means for selecting one of the intensity level error registers according to a signal representative of the visible effect of the intermediate intensity level that it is desired that a particular pixel should produce; means for setting the particular pixel to either a first or second intensity level depending on the value contained in the selected error register; and means for updating the value contained in the selected register by adding a value corresponding to the first or second intensity level to which the particular pixel was set, subtracting a value corresponding to the intermediate intensity level, the visible effect of which it is desired that the particular pixel should produce, and adding a random number.
The provision of a separate error register for each intensity level it is desired to reproduce enables narrow vertical structures to be reproduced without requiring the use of two dimensional error propagation schemes which require the storage of the propagation errors of a complete line. In effect a one dimensional error propagation scheme is implemented for each of the intensity levels it is desired to reproduce. Thus if it is desired to reproduce eight gray scale levels then six error registers will be needed. Clearly separate error registers are not needed for black and white.